Only allow safe cache configurations on ARMv6 and later

Currently, Linux doesn't generate correct page tables for ARMv6 and
later cores if the cache policy is different from the default one (it
may lead to strongly ordered or shared device mappings). This patch
disallows cache policies other than writeback and the
CPU_[ID]CACHE_DISABLE options only affect the CP15 system control
register rather than the page tables.
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm/mm/Kconfig

@@ -627,7 +627,7 @@ config CPU_DCACHE_SIZE
 
 config CPU_DCACHE_WRITETHROUGH
 	bool "Force write through D-cache"
-	depends on (CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || CPU_ARM1020 || CPU_V6) && !CPU_DCACHE_DISABLE
+	depends on (CPU_ARM740T || CPU_ARM920T || CPU_ARM922T || CPU_ARM925T || CPU_ARM926T || CPU_ARM940T || CPU_ARM946E || CPU_ARM1020) && !CPU_DCACHE_DISABLE
 	default y if CPU_ARM925T
 	help
 	  Say Y here to use the data cache in writethrough mode. Unless you

arch/arm/mm/mmu.c

@@ -114,6 +114,10 @@ static void __init early_cachepolicy(char **p)
 	}
 	if (i == ARRAY_SIZE(cache_policies))
 		printk(KERN_ERR "ERROR: unknown or unsupported cache policy\n");
+	if (cpu_architecture() >= CPU_ARCH_ARMv6) {
+		printk(KERN_WARNING "Only cachepolicy=writeback supported on ARMv6 and later\n");
+		cachepolicy = CPOLICY_WRITEBACK;
+	}
 	flush_cache_all();
 	set_cr(cr_alignment);
 }
@@ -248,13 +252,15 @@ static void __init build_mem_type_table(void)
 	int cpu_arch = cpu_architecture();
 	int i;
 
+	if (cpu_arch < CPU_ARCH_ARMv6) {
 #if defined(CONFIG_CPU_DCACHE_DISABLE)
-	if (cachepolicy > CPOLICY_BUFFERED)
-		cachepolicy = CPOLICY_BUFFERED;
+		if (cachepolicy > CPOLICY_BUFFERED)
+			cachepolicy = CPOLICY_BUFFERED;
 #elif defined(CONFIG_CPU_DCACHE_WRITETHROUGH)
-	if (cachepolicy > CPOLICY_WRITETHROUGH)
-		cachepolicy = CPOLICY_WRITETHROUGH;
+		if (cachepolicy > CPOLICY_WRITETHROUGH)
+			cachepolicy = CPOLICY_WRITETHROUGH;
 #endif
+	}
 	if (cpu_arch < CPU_ARCH_ARMv5) {
 		if (cachepolicy >= CPOLICY_WRITEALLOC)
 			cachepolicy = CPOLICY_WRITEBACK;